3 Exciting Treadmill Workouts That Only Take 20 Minutes

These back-to-basics workouts will get your heart racing and calories burning.

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Why the craze?After so many off-the-wall trends (Shake Weight, anyone?), classes like Equinox's new TreadZone are taking a back-to-basics approach. "Walking fast or jogging is one of the most natural and easy things for the human body to do," says Gregory Florez, a trainer, executive coach, and spokesperson for the American Council on Exercise. "And there's no better cardio workout." Interval training, which is the basis of all three of these routines, is proven to burn the most calories and get you in tip-top shape fast as can be.

Wait a second!Before you get started, reduce the likelihood of soreness (ow) and injury (more ow) by doing a few stretches before you really start moving. Florez recommends stretching both hamstrings for at least 30 seconds by standing with your feet hip-width apart and then touching your toes; stretching quads for at least 30 seconds each by standing on one leg and pulling each ankle toward your butt; and stretching your calves for at least 30 seconds each by standing, leaning your hands against a wall, and then stretching one foot behind you on the ground and raising your heel.

And while the person next to you may look like they're flying, what matters most for your workout is your heart rate — in other words, your level of exertion. Aim to be at 65 to 75 percent during the easy to moderate parts of the routine, and to reach 80 percent during the toughest moments.

No heart-rate monitor? No problem. Just remain aware of how much you're sweating, how uncomfortable you feel, and, most importantly, how hard you're breathing.

The Wake-Up CallLevel: Beginner

>> The first 2 minutes: Warm up by walking at a comfortable pace.>> Minutes 2 to 5: Crank up the speed until you're power walking — you should be able to carry on a conversation, but need to take a breath during every other sentence.>> Minutes 5 to 7: Speed up to a slow jog.>> Minutes 7 to 10: Go back to walking briskly.>> Minutes 10 to 12: It's jogging time!>> Minutes 12 to 15: OK, walk again, but keep up the pace.>> Minutes 15 to 20: This is your cool-down. Gradually go from a brisk to a slow walk.

As you get stronger, add more jogging phases and/or make these longer until you've reached 40 minutes of total walking and running.

The Sweat SessionLevel: Intermediate

>> The first 2 minutes: Walk at a comfortable pace to warm up.>> Minutes 2 to 5: Turn up the speed until you're walking briskly — you should be able to speak just fine, but need to take a breath during every other sentence.>> Minutes 5 to 15: Increase the incline by one percent every minute. By minute 15, you should be breathing heavily and have trouble chatting.>> Minutes 15 to 17: Keep the same pace, but gradually decrease the incline back to zero.>> Minutes 17 to 20: During your cool-down, slow steps until you're walking at an easy pace.

As you become more fit, double up. Increase the incline by two percent each minute and/or walk at each incline level for two minutes.

The Belly BurnerLevel: Advanced

>> The first minute: Warm up by walking at a comfortable pace.>> Minutes 1 to 3: Amp up your speed until you're walking briskly.>> Minutes 3 to 5: Raise the speed until you're jogging at a pace at which you can carry on a conversation but need to take a breath during every other sentence.>> Minutes 5 to 6: Start running fast, to the point where you are a little uncomfortable and breathing hard.>> Minutes 6 to 8: Lower the speed until you're jogging at a medium pace.>> Minutes 8 to 10: Return to running at a fast pace that makes it difficult to breathe.>> Minutes 10 to 12: Go back to jogging at a medium pace.>> Minutes 12 to 15: Time to run again!>> Minutes 15 to 17: Okay, you can jog. Breathe!>> Minutes 17 to 19: Slow down, going from a slow jog to a brisk walk.>> Minutes 19 to 20: As you cool down, walk slowly.

"If you don't jog regularly, work your way up to this routine," says Florez. Already a pro? Increase the number, length, and/or speed of the jogging intervals.